This article examines the cause of school type effects upon gaining a first class degree at Oxford University, whereby for a given level of secondary school performance, private school students perform less well at degree level. We compare the predictive power of an aptitude test and secondary school grades (GCSEs) for final examination performance, using data from the Oxford Admissions Study. Both metrics are predictive of final degree performance but the school effects are only statistically robust for arts students. Private school students perform less well in final examinations relative to their GCSE results when compared with state school students, but they do not under perform relative to their aptitude test scores or in gross terms. It is therefore argued that teaching effects, associated with private school students, distort secondary school grades as an indicator of academic potential in higher education when compared to state school students. (Contains 6 tables, 6 figures and 4 notes.)